Shrimp trawling basically involves the deployment from a fishing vessel of a continuous, large, substantially arced trawl net, suspended between side paravane members (trawl boards) which are shaped so that the forces of trawl dragging cause the net to extend and sweep a substantial swath of ocean space during trawling. Because the desired catch is shrimp, a relatively small ocean crustacean, the trawl nets are considerably finer than are commonly encountered in the capture of species such as fish. Often the trawl is towed for a significant period of time, upwards of twenty-four hours.
In order to reduce the workload involved in periodically emptying the trawl net, the net is sewn so that it tapers into a substantially stronger collecting section, located in the aft middle of the net, and so positioned that it is merely necessary to raise this section or "cod end" to dump the catch on board the shrimp trawler. The main trawl body remains immersed and the trawl continues during this process.
The speed of the tow captures in the nets more sea creatures than the desired shrimp; this is exacerbated by the fact that shrimp is also the preferred food of numerous marine species including fish and marine reptiles, especially sea turtles. The overall trawl speed is such, when trawling for shrimp, that average marine fish can swim against the speed of the flow, and in fact fish can readily enter into the cod end and swim out of the cod end against the force of the flow stream.
It is also common that a trawl will be dragged along the bottom surface of a body of water, and the trawl net is provided with special accessories, e.g., trawl boards, to insure close travel along the bottom surface.
For best results, the cod end should be free floating, with only the bottom leading edge of the trawl in contact with the bottom. However, such use gradually picks up debris, including rocks, in the net, weighing it down and causing it to drag along the bottom. This not only reduces the efficiency of the trawl; it damages the net and seriously increases the costs of shrimping along rocky bottomed waters.
It has become a matter of public policy to provide a device or apparatus permitting the escape or exclusion of captured sea turtles. Such a device of necessity must also be relatively inexpensive as it must be procured by all shrimp trawling fishermen in a quantity sufficient to equip all trawl nets. The device must additionally not permit or cause any substantial loss of shrimp, although it would be desirable that the device exclude, if possible, fish which are predatory on shrimp during the trawling process.
The National Marine Fisheries, a branch of the United States Government, has identified and certified four separate turtle excluder devices for insertion within the cod end of a shrimp trawl net. The exact description of these devices and their approval is being codified in 50 C.F.R. 217, 222, and 227, and is the current subject of a proposed rule making.
The basic construction of all of the turtle excluder devices is to provide an angled rigid array located between the trawl body and the cod end, fastened to the trawl nets, which is sufficient to admit the passage of shrimp but which acts a bar to the further passage of larger sea creatures including specifically sea turtles. The angled effect further provides for deflection of the turtle before the cod net. One of the turtle excluder devices provides for a vertically angled bar array with the forward (in the sense of in the direction of trawl) end being at an upper portion and the after end being at a lower portion, and providing a sufficiently large hole (turtle hole) within the trawl net for the turtle to escape. While it is believed that this device will also provide for the elimination of fish, it is also considered that the device has an excessive shrimp loss rate as the shrimp are free to exit the turtle hole.
A second device uses vertically ascending parallel PVC pipe, providing for openings between each section of pipe for the passage of shrimp. In this device the angle is from a forward lower end to an aft upper end and a loose section of net is provided which is considered adequate to allow the turtle to push free. The loose net, being unsecured, has a definite wave or flapping action during tow and is considered to provide again for an excessive loss of shrimp.
The third such device provides for a flat trap door hinged to the turtle extraction bars and covered with a section of net. Again, the opening of the door and the fluttering of the door is considered likely permitting excessive loss of shrimp and in addition the extending portion of the net along the trap door is considered to provide significant problems in regards to catching or snagging on the shell of a turtle.